1. Field of the Invention
The present invention relates to an electrolyte of a lithium secondary battery and a lithium secondary battery comprising the same, and more particularly, to an electrolyte of a lithium secondary battery that prevents the battery from swelling while maintaining the electrochemical properties of the battery, and a lithium secondary battery comprising the same.
2. Description of the Related Art
Due to recent trends toward more compact and lighter portable electronic equipment, there has been a growing need to develop a high performance and large capacity battery to power the portable electronic equipment. In particular, there has been extensive research to provide lithium secondary batteries with good safety characteristics and improved electrochemical properties. Lithium secondary batteries use lithium metal oxides as positive active materials, and lithium metals, lithium-containing alloys, or crystalline or amorphous carbons, or carbon-containing composites as negative active materials.
The average discharge voltage of a lithium secondary battery is about 3.6 to 3.7 V, which is higher than the average discharge voltage of other alkali batteries, Ni-MH batteries, Ni—Cd batteries, and the like. An electrolyte that is electrochemically stable in the charge and discharge voltage range of 0 to 4.2 V is required to generate a high driving voltage. As a result, a mixture of non-aqueous carbonate-based solvents, such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, and the like, is used as an electrolyte. However, such an electrolyte has significantly lower ion conductivity than an aqueous electrolyte that is used in a Ni-MH battery or a Ni—Cd battery, thus resulting in the deterioration of battery characteristics during charging and discharging at a high rate.
During the initial charge of a lithium secondary battery, lithium ions, which are released from the lithium-transition metal oxide positive electrode of the battery, are transferred to a carbon negative electrode, where the ions are intercalated into the carbon. Because of lithium's high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, and the like, thus forming a thin film on the surface of the negative electrode. The film is referred to as an organic solid electrolyte interface (SEI) film. The organic SEI film formed during the initial charge not only prevents the reaction between the lithium ions and the carbon negative electrode or other materials during charging and discharging, but also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents disintegration of the structure of the carbon negative electrode, which is caused by co-intercalation of organic solvents having a high molecular weight, along with solvated lithium ions, into the carbon negative electrode.
Once the organic SEI film is formed, lithium ions do not react again with the carbon electrode or other materials, so that an amount of lithium ions is maintained. That is, the carbon of the negative electrode reacts with an electrolyte during the initial charging, thus forming a passivation layer such as an organic SEI film on the surface of the negative electrode so that the electrolyte solution no longer decomposes, and stable charging and discharging are maintained (J. Power Sources, 51(1994), 79–104). Due to the above reasons, in the lithium secondary battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained.
However, gases are generated inside the battery due to decomposition of a carbonate-based organic solvent during the organic SEI film-forming reaction (J. Power Sources, 72 (1998), 66–70). The gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, and the like, depending on the type of non-aqueous organic solvent and the negative active material used. The thickness of the battery increases during charging due to the generation of gas inside the battery.
The passivation layer is slowly disintegrated by electrochemical energy and heat energy, which increases with the passage of time when the battery is stored at a high temperature after being charged. Accordingly, a side reaction in which an exposed surface of the negative electrode reacts with surrounding electrolyte occurs continuously. The internal pressure of the battery increases with the generation of gases, inducing the deformation of prismatic batteries or pouch batteries. As a result, regional differences in the cohesion among electrodes inside the electrode assembly (positive and negative electrodes, and separator) of the battery occur, thus deteriorating the performance and safety of the battery, and making it difficult to mount the lithium secondary battery set into electronic equipment.
To improve low temperature characteristics, a lithium secondary battery using liquid electrolyte uses an organic solvent with a low boiling point which induces swelling of a prismatic or pouch battery during high temperature storage. As a result, reliability and safety of the battery are deteriorated at a high temperature.
Accordingly, extensive research into a liquid electrolyte with a high boiling point is needed. An example of an electrolyte with a high boiling point includes an ester solvent, such as gamma butyrolactone (GBL). When using 30 to 70% of an ester solvent, cycle life characteristics are significantly deteriorated, and therefore it is difficult to apply an electrolyte with an ester solvent to batteries. It has been suggested that, as an electrolyte with a high boiling point, a mixture of gamma butyrolactone/ethylene carbonate (7/3) may be used, and a boron-coated mesocarbon fiber (MCF) as a negative active material may be used to reduce swelling at a high temperature and improve cycle life characteristics (Journal of Electrochemical Society, 149(1) A(9)–A12(2002)). However, when an uncoated carbonaceous material is used as a negative active material, cycle life characteristics are deteriorated even when an electrolyte with a high boiling point is used. It should be noted that ethylene carbonate need not be used in all aspects, i.e., GBL may be used alone as the solvent.
To solve the problem of deterioration of cycle life characteristics, an electrolyte including vinylene carbonate has been developed (U.S. Pat. Nos. 5,352,458 and 5,626,981). However, sufficient improvement of cycle life characteristics has not been obtained.
U.S. Pat. No. 5,529,859 discloses an electrolyte that is prepared by adding a halogenated organic solvent, e.g., chloroethylene carbonate, to propylene carbonate, resulting in improvement of battery performance and capacity. U.S. Pat. No. 5,571,635 discloses an electrolyte that is prepared by adding a halogenated organic solvent, e.g., chloroethylene carbonate to a mixture of propylene carbonate and ethylene carbonate, resulting in improvement of battery performance and capacity. The propylene carbonate has a high viscosity. When propylene carbonate is applied to a battery along with a crystalline carbon such as graphite, the propylene carbonate is inserted into a carbon layer of the negative electrode and is decomposed to generate propylene gases and lithium carbonate, resulting in a reduction of battery capacity and an increase of irreversible capacity. In the above U.S. Patents, propylene carbonate and chloroethylene carbonate are used in a mixed ratio of 1:1, but with the above mixed ratio, the wetability of the electrolyte is low.